1. Field of the Invention
This invention relates to sprockets and gears, and in particular to a construction for sprockets and gears which is made using dissimilar compatible powder metals.
2. Discussion of the Prior Art
Internal combustion engines must ensure that the piston motion which compresses the air/fuel mixture is coordinated with the intake and exhaust valves opening and closing by means of a timing system. The most common timing system utilizes two sprockets with teeth encompassed by a metal link chain. The crankshaft sprocket drives the chain which in turn applies torque to the camshaft sprocket, thereby turning it in unison. Typically, the camshaft sprocket is significantly larger than the crankshaft sprocket, to effect a speed reduction.
Camshaft sprockets are usually made from a metal stamping or machined casting of aluminum alloy or cast iron. Modern high output engines demand greater precision and improved endurance including tooth strength and wear resistance. An additional requirement is quietness, referred to as NVH quality (noise vibration and harshness).
In recent years, a new technique of manufacture is powder metallurgy (P/M). This involves the use of iron and other powders which are blended and then compacted into a preform shaped like the cam sprocket. A thermal treatment called sintering causes the compacted particles to bond together metallurgically forming a structural component. The P/M process has the benefit of large volume precision component manufacture.
In the case of high functional demands, for example a sprocket or gear, a P/M part has to be made to a high density. This generally requires a process sequence involving powder compaction, sintering, repressing and finally induction hardening. The high alloy and high density result in high cost in production and high weight, particularly in a large sprocket or gear. In addition, high density imparts a high modulus of elasticity to the P/M alloy which transmits noise (i.e. "rings" when subjected to mechanical vibration).